Programmed death-ligand 1 (PD-L1) is a protein that has been implicated in the suppression of immune system responses during chronic infections, pregnancy, tissue allografts, autoimmune diseases, and cancer. PD-L1 regulates the immune response by binding to an inhibitory receptor, known as programmed death 1 (PD-1), which is expressed on the surface of T-cells, B-cells, and monocytes. PD-L1 negatively regulates T-cell function also through interaction with another receptor, B7.1 (also known as B7-1 or CD80). Formation of the PD-L1/PD-1 and PD-L1/B7.1 complexes negatively regulates T-cell receptor signaling, resulting in the subsequent downregulation of T cell activation and suppression of anti-tumor immune activity. PD-L1 is overexpressed in many cancers, including a wide variety of solid tumors, such as bladder, breast, colon, lung, melanoma, ovarian, salivary, stomach, and thyroid tumors. PD-L1 overexpression in tumor cells may advance tumor invasion and is often associated with poor prognosis.
Given the role of PD-L1 in cancer development and immune system regulation, additional tools to detect the presence of PD-L1, for example for diagnosis and/or patient selection, are desirable.